Fix salt.states.selinux._refine_value()

This function was not properly handling True/False values, causing the
state not to work when one of these values is used. This makes the
example in the docstring fail.

Also cleaned up/simplified _refine_mode.

Fixes #5912.
Merge pull request #5917 from terminalmage/issue5912

Fix salt.states.selinux._refine_value()